Wave height measuring device

ABSTRACT

The invention relates to a wave measuring instrument for use on a dynamically positioned or tethered floating vessel whereby to continuously monitor wave height, wave period and vertical heave of the vessel as the vessel responds to waves, swell and currents. The instrument comprises a measuring device connected to the vessel hull in a substantially submerged position whereby to continuously record the distance between the instrument and the ocean floor as the vessel heaves. A second measuring means automatically gauges and records the normally varying distance between the instrument and the water&#39;s surface, which distance results from the usual presence of wave action whether the latter be gradually rolling or relatively turbulent. 
     The respective measurements thus obtained are thereafter integrated and compensated for to provide a relatively accurate record of the actual wave height between trough and crest, the wave period, and the vertical movement or heave of the vessel.

This is a continuation of application Ser. No. 248,995, filed May 1,1972, now U.S. Pat. No. 3,910,111

BACKGROUND OF THE INVENTION

There presently exist a number of ways for determining the height andperiod of waves in an open stretch of water. Such measurements can beachieved most readily either through visual means, or through the use ofappropriate electronic instrumentation. For accuracy however, suchmeasuring devices must be positioned on a fixed structure which does notrespond to the waves.

On the other hand, such instruments cannot be utilized on floatingvessels as their effectiveness is contingent on the stability of thevessel and the condition of the seas thereabout. Where the seas are in aturbulent condition, the consequent vessel motion is relatively severe.The difficulty therefore of obtaining an accurate wave height and periodmeasurement is compounded if not precluded.

In the instance of floating type mobile offshore drilling units adaptedto drill submerged wells for gas, oil and other fluids, it is highlydesirable to continuously monitor the condition of the open seas and theresponse of the unit to the sea conditions. Said monitoring is requiredto establish future design and operating conditions, and to determinewhen to shut down drilling operations and disconnect from the well, allof which bears upon the success of the operation and the safety of thepersonnel involved.

For example, in the drilling of any offshore well from a floatingvessel, the latter is subject to continuous rolling, heaving andpitching motion as a result of wind, wave and swell action. It iseconomically desirable to maintain a continuous drilling operation.Thus, the drilling equipment must be adapted to adjust in response tosuch movement of the vessel and especially to the vertical heaving ofthe latter.

In this respect such vertical movement can be accommodated by the use ofcompensating equipment incorporated within the drill string to minimizethe effect of the moving vessel on the drill bit. Howwever, it hasbecome practical and even desirable to compensate for said heaving bythe use of heave compensating equipment carried directly on the ship.

In the instance of the latter, the function of such compensatingequipment is to in effect adjust the height of the travelling block asthe ship rises and falls thereby maintaining a constant tension on thedrill pipe. The actuating system in such an instance comprises normallya pressure source which can be actuated to rapidly raise or lower thesaid travelling block and maintain the relative position thereof as thevessel passes through its periodic undulations in response to waveaction.

Therefore, to more effectively utilize the compensating mechanism, it isdesirable to determine in advance the approximate wave pattern of thewater as the ship rides thereon. With this pattern determined, the drillstring compensating apparatus can be programmed or actuated in advance,whereby to anticipate movement of the vessel, and consequently adjustthe latter at the correct timing and stroke length.

It is further desirable, and one of the objectives of the invention, toeffectively regulate such actuating equipment, in response to therepeating wave pattern of the water. The latter is thus constantlycalculated as a determinant for the programming of the actuatingequipment.

In achieving the objects of the invention which is to provide asimplified, effective wave height and period measuring device and aheave measuring device, the latter includes a mounting means on a vesselfor holding vertically aligned wave measuring instruments. Eachinstrument is operably mounted or positioned so as to be readilyadjustable in response to both vertical heaving and rolling and pitchingof the vessel. Thus, the actual measuring devices are maintained in asubstantially constant vertical orientation and respond only to thevertical motions of the vessel.

DESCRIPTION OF THE DRAWINGS

FIG. 1 represents an elevation view of a semi-submersible drillingvessel adapted to operate in offshore waters for drilling subsea wells.

FIG. 2 is a segmentary view of a portion of the platform shown in FIG.1.

FIG. 3 is a diagrammatic representation of a floating vessel whereon thepresent device is utilized.

FIG. 4 is a segmentary view on an enlarged scale of a wave measuringdevice of the type contemplated.

FIG. 5 is similar to FIG. 4.

FIG. 6 illustrates an alternate embodiment of the devices shown in FIGS.4 and 5.

Referring to the drawings, FIG. 1 illustrates an embodiment of anoffshore platform 10 of the type contemplated. The platform itselfcomprises a deck 11 which is normally positioned above the surface 15 ofthe body of water 12 by a controllably buoyant support or understructure13. A measuring means or instrument package 14 is so mounted to asubmerged portion of understructure 13 to monitor the water conditionsand to record measurable data.

Referring to FIG. 2, instrument package 14 comprises in part, means 17for generating a relatively low frequency sound wave which is directeddownwardly toward the ocean floor 16. After being reflected from thelatter, at least a portion of the sound wave is reflected upwardlythrough the water and received by a sound receiving device.

Similarly, a second sound wave is generated and directed verticallyupwardly by a second sound generating means 18, toward the water'ssurface 15 where it will strike the air-water interface and a portionthereof being reflected downwardly to be received and recorded.

Said first signal source and transmitter 17 can comprise a standard formof electronic depth gauge or similar device. One embodiment of such aninstrument includes a sonic signal generator and transmitter, commonlyreferred to as a transducer. Said instrument, at predetermined intervalsof time, produces and receives a sound wave. The latter thus serves asboth emitter for the downwardly directed wave, and as a receiver orcollector for the returning portion of the echoed wave.

Operationally, the water depth through which the transmitted wave passesis determined as a function of the time elapsed between transmission andreception of the signal or wave. In any instance, the time of passage ofthe sound wave will provide an instantaneous determination of the depthof water beneath the floating platform.

Simultaneously with the depth measuring signal, the height of the watercolumn immediately above instrument package 14 is determined. As noted,the latter can comprise a second sound generating and transmitting means18, or it can assume the embodiment of a pressure sensitive instrument.While a barometric pressure responsive device is applicable to theinstant member, there are a number of commercially available devicesadapted to rapidly and accurately measure the height of a vertical watercolumn above the device. The latter measured distance will serve torepresent the distance between the water's surface and the instrumentpackage at any particular instance during a wave period. Said member isthus necessarily rapidly responsive to pressure changes and variations.Such a requirement of responsiveness is essential since normally theplatform will be subjected to not only continuous wave action, but alsoto long, gradual swells which tend to rise and fall in a set pattern.

The mounting means for stabilizing the under water instrument package ina vertical orientation can assume the arrangement shown in FIG. 4. Thetransducers 17 and 18 would be connected by a shaft 19 which isconnected to gimbal axles 20 and 21. The gimbal supported transducers 17and 18 would be enclosed inside a fluid filled sphere 22. The sphere 22would shield the gimbal mounted transducers 17 and 18 from currentforces that are generated by wave and current induced vessel motions.The shielding of the transducers 17 and 18 will allow all measurementsto be made in a vertical plane as the vessel 10 rolls and pitches fromwave and current action.

An alternate means of mounting the under water instrument package isshown in FIG. 5. The transducers 17 and 18 would be mounted in a fixedposition. A roll and pitch indicator 23 to measure vessel motions fromvertical would be mounted on the shaft 19 to which the transducers 17and 18 are connected. The transducers 17 and 18 would measure inclineddistances that would have to be corrected based on angularity introducedby roll and pitch of the vessel 10.

The true height of a wave is here represented by the distance X whichdenotes the distance between the crest and trough of a particular wave.By determining the respective distances A-B as well as A'-B', theoverall difference in said figures will provide the desired X distanceindicating the wave height.

The rapid integration of A plus B distances to find X, can of course beachieved mathematically as the respective distances A and B aredetermined for a particular point. However, preferably said figuresrepresented by X can best be recorded automatically through a suitabledouble pen recorder which simultaneously records distances A and B oneabove the other and continuously fed sheet of recording paper.

Other modifications and variations of the invention as hereinbefore setforth may be made without departing from the spirit and scope thereof,and therefore, only such limitations should be imposed as are indicatedin the appended claims.

We claim:
 1. Method for measuring wave height in a body of water from amarine structure floatably positioned in said body of water andsubjected to vertical heaving resulting from movement of said water,said method comprising the steps of:measuring a first distance between areference point on said floating structure and the floor of said body ofwater, simultaneously measuring a second distance between said referencepoint to the interface of the water and the atmosphere above saidreference point, repeating the above steps when said structure is at thecrest and trough respectively of a wave to be measured, correlating thedistances obtained when the structure is at the wave trough and crestwhereby to determine the height of said wave and the period of timewhich elapses between successive wave height measurements whereby toestablish the period of time which occurs between successive waves. 2.The combination with a marine structure adapted to be floatablypositioned in an offshore body of water in which high waves andturbulent water conditions are prevalent, and which marine structure isfree to be displaced vertically in response to the forces of wind andwater acting thereagainst, of a wave height measuring devicecomprising:first measuring means depending from said structure, beingdisposed beneath the surface of said body of water and being actuatableto generate a first signal for gauging the vertical distance betweensaid first measuring means and the floor of said body of water, secondmeasuring means depending from said structure and disposed beneath thewater's surface, said second measuring means being actuatable togenerate a second signal to gauge the vertical distance between saidsecond measuring means and said water's surface, means for integratingsaid respective first and second signals whereby to determine thevarying height of said water's surface, and pitch indicator meanscooperative with said respective first and second measuring means, andbeing operable to measure the degree of vertical displacement existingbetween said first and second measuring means in response to movement ofsaid marine structure.